Temporal redundancy can be exploited using prediction of pixel values in the current frame by using a previous frame. Pixel prediction is an important part of video coding standards such as H.261, H.263, MPEG-4 and H.264 [1]. In H.264 there are two pixel prediction methods utilized namely intra and inter frame prediction. Intra frame prediction gives a spatial prediction of a block in frame currently being encoded using previously reconstructed pixels from the same frame. Inter frame prediction gives a temporal prediction of the current block using a corresponding but displaced block in a previously reconstructed frame, e.g. reference block. Inter frame prediction can also use a weighted average of two inter frame predictions. The prediction error compared to an original is then transform coded and quantised. A reconstructed frame can then be generated by adding the predictions and the coded prediction errors. A loop filter is then applied to reduce coding artefacts before storing the frame in a reference frame buffer for later use by Inter frame prediction.
In order to increase the coding efficiency of current video coding standards adaptive filters for inter frame prediction [2] or loop- or post filtering [6] have been proposed. In Inter frame prediction typically several sub-pel motion specific adaptive filters are used for each frame. If many adaptive filter coefficients are used per frame there will be a substantial overhead for the representation of adaptive filter coefficients. Typically reference adaptive filter coefficients corresponding to a Wiener filter are subtracted from the adaptive filter coefficients to achieve small difference filter coefficients [2]. Then the difference filter coefficients are quantised. Prediction between quantised difference filter coefficients from a nearby sub-pel position is exploited in some cases [4]. Prediction in the context of adaptive filter coefficients may be interpreted as estimation of adaptive filter coefficients. Filter symmetry may be exploited for the prediction of adaptive filter coefficients [3,4,5].
Since the adaptive filter coefficients are determined in the encoder and provided to the decoder they add on the overhead of the total bit rate. The relative overhead of the adaptive filter coefficients to the total bit rate depends on the actual bit rate used. For low resolution video the overhead can be very high, more than 10%.